The present invention relates to a coding system which is effective for efficient transmission or storage of a multilevel-gradated/facsimile signal.
One method that has been employed for representing a multilevel-gradated picture as of a photograph in a shaded form by the binary terms is an ordered dither method. This is a method which represents the degree of shading of the picture in terms of the area ratio between black picture elements and white picture elements.
On the other hand, Modified Read (MR) coding system is standardized by the International Telegraph and Telephone consultative Committee (CCITT). According to this system, when an information change picture element (which is a picture element having changed from white to black or vice versa and will hereinafter be referred to as a "change picture element") is coded, its address relative to a preceding change picture element on the same scanning line or on a preceding scanning line is coded. This system achieves a high compression rate of a two-level facsimile signal. In case of coding the aforesaid dither picture by this system, however, a high compression rate cannot be expected because the dither picture has an appreciably large number of change picture elements as compared with the two-level facsimile signal.